1. Field of the Invention
The present invention relates to a shadow mask for a cathode ray tube. More particularly, the present invention relates to a shadow mask that is suitable for use in a cathode ray tube having a large and flat panel, and to a cathode ray tube using the shadow mask.
2. Description of the Related Art
When a cathode ray tube (CRT) is applied as the main element in a color television, a shadow mask used in the CRT performs a color selection function by directing electron beams emitted from an electron gun such that the electron beams correctly land on a phosphor screen. The shape of such a shadow mask is determined by the size and shape of the CRT panel, which is a front glass portion of the CRT. The shadow mask typically has a curvature radius of R=2,000 mm in a diagonal direction of the shadow mask (assuming the shadow mask is substantially rectangular). However, with consumer preference for larger and flatter screens in recent times, it is necessary to increase the size and flatten the shadow mask when used in such a CRT.
In practice, when a shadow mask is applied to a large-sized CRT using a panel with a flat external surface and a curved inner surface, a shadow mask is used that matches the size of the panel but is curved identically to conventional shadow masks. If the shadow mask is both enlarged and its curvature radius increased, the mask becomes structurally weak. This causes many problems. For example, if the curvature radius of the shadow mask is 1.6 R or greater, the shadow mask may be easily deformed by an external shock of a predetermined force or greater. Such deformation of the shadow mask significantly reduces the quality of the CRT.
Further, the shadow mask becomes vulnerable to howling, a phenomenon caused by transfer of vibration, if increased in size and made flatter. For example, if the CRT having a shadow mask is used in a large color television, the shadow mask vibrates as a result of sound generated by the speakers. With the increase in the size of the shadow mask, howling becomes even more of a problem since the shadow mask becomes structurally weak.
To remedy the problem of deformation of the shadow mask as a result of receiving a shock, a thickness of the panel to which the shadow mask is mounted is adjusted. In particular, peripheral portions of the panel are made greater in thickness than a center portion of the same (approximately two times thicker or more), and the shadow mask is formed having a corresponding radius such that damage caused by external shock may be reduced.
However, by this formation of the panel in which the peripheral portions are made thicker than the center portion thereof, the overall weight of the CRT increases. This makes manufacture more difficult and may inconvenience users when moving the device.
In addition, if an optimum thickness ratio between the center and peripheries of the panel in consideration of the shock characteristics of the shadow mask is not able to be obtained, that is, if the thickness at peripheries is too great compared to the thickness of the center portion of the panel, it becomes necessary to form a coating film, which adjusts transmissivity, on a front surface of the panel in order to prevent a deterioration in contrast characteristics of the CRT caused by the transmissivity of the glass forming the panel. This extra step of forming the coating film complicates the overall manufacturing process, ultimately increasing CRT unit costs.
Therefore, a reduction in the shock characteristics of the shadow mask when making the panel flat and increasing the size of the panel and shadow mask, as well as the complication in the manufacture of the CRT resulting from attempts to improve its brightness characteristics are contrary to efforts at providing a superior CRT.